Field of the Invention
The present disclosure relates to techniques for detecting and identifying objects on a touch surface.
Description of the Related Art
To an increasing extent, touch-sensitive panels are being used for providing input data to computers, electronic measurement and test equipment, gaming devices, etc. The panel may be provided with a graphical user interface (GUI) for a user to interact with using e.g. a pointer, stylus or one or more fingers. The GUI may be fixed or dynamic. A fixed GUI may e.g. be in the form of printed matter placed over, under or inside the panel. A dynamic GUI can be provided by a display screen integrated with, or placed underneath, the panel or by an image being projected onto the panel by a projector.
There are numerous known techniques for providing touch sensitivity to the panel, e.g. by using cameras to capture light scattered off the point(s) of touch on the panel, by using cameras to directly observe the objects interacting with the panel, by incorporating resistive wire grids, capacitive sensors, strain gauges, etc. into the panel.
In one category of touch-sensitive panels known as ‘above surface optical touch systems’ and known from e.g. U.S. Pat. No. 4,459,476, a plurality of optical emitters and optical receivers are arranged around the periphery of a touch surface to create a grid of intersecting light paths (otherwise known as detection lines) above the touch surface. Each light path extends between a respective emitter/receiver pair. An object that touches the touch surface will block or attenuate some of the light paths. Based on the identity of the receivers detecting a blocked light path, a processor can determine the location of the intercept between the blocked light paths.
For most touch systems, a user may place a finger onto the surface of a touch panel to register a touch. Alternatively, a stylus may be used. A stylus is typically a pen shaped object with at least one end configured to be pressed against the surface of the touch panel. An example of a stylus according to the prior art is shown in FIG. 2. Use of a stylus 60 may provide improved selection accuracy and pointer precision over a simple finger touch. This can be due to the engineered stylus tip 62 providing a smaller and/or more regular contact surface with the touch panel than is possible with a human finger. Also, muscular control of an entire hand in a pen holding position can be more precise than a single finger for the purposes of pointer control due to lifelong training in the use of pens and pencils.
PCT/SE2016/051229 describes an optical IR touch sensing apparatus configured to determine a position of a touching object on the touch surface and an attenuation value corresponding to the attenuation of the light resulting from the object touching the touch surface. Using these values, the apparatus can differentiate between different types of objects, including multiple stylus tips, fingers, palms. The differentiation between the object types may be determined by a function that takes into account how the attenuation of a touching object varies across the touch surface, compensating for e.g. light field height, detection line density, detection line angular density etc.
For larger objects applied to the touch surface, such as palms and board erasers, it is possible to use an interaction map of the touch surface to determine an approximate shape of the object. For example, where an optical IR touch sensing apparatus is used, an attenuation map may be generated showing an area on the touch surface where the light is highly attenuated. The shape of an attenuated area may then be used to identify the position and shape of the touching object. In FIG. 5, an example attenuation map of a board eraser is shown. In a technique known according to the prior art, a rough shape of the eraser can be determined by identifying all points with an attenuation above a threshold value. An approximate centroid and orientation of the eraser may then be determined using the image moments of the identified points. Such techniques are described in “Image analysis via the general theory of moments” by Michael Reed Teague. Once the centroid and orientation of the board eraser are determined, width and height of the board eraser can be found by determining the extent of the identified pixels in the direction of the orientation angle and the normal of the orientation angle.
A known problem with this technique is that larger objects placed onto an optical IR touch sensing apparatus, such as those described above, may cause severe distortion to the touch signal. Many systems use a large number of detection lines and a high density detection line grid density across the touch surface to ensure high touch resolution. Whilst a relatively small number of detection lines may be interrupted by a finger or stylus, a larger object, such as a palm or board eraser, may disrupt a relatively large number of detection lines and cause artefacts in the processed touch signal. This may result in a distorted and noisy interaction map, making the position, size, and orientation of the larger object impossible to detect accurately and reliably.
Therefore, what is needed is a way of improving the identification of position, size, and orientation of large objects touching an optical touch system that mitigates the above problem.